Reactive polyolefin compositions that can be used as hot melt adhesives (also referred to as hotmelts) have been known for a long time. They usually include silane-grafted polyolefin prepolymers obtained by mixing with other polymers with resins. Such adhesives develop a high initial strength immediately after their application due to cooling, and they achieve their final properties, for example, heat resistance and resistance to environmental influences, due to the gradually occurring “curing,” i.e., the chemical reaction of the silane groups with atmospheric moisture.
For film laminating, for example, for vacuum deep-draw processes in car manufacturing, polyurethane (PUR) adhesives in various forms are predominantly used. PUR adhesives are used predominantly in dispersed or dissolved form. In these adhesive systems, due to the high molecular weight of the polyurethane, it is necessary to work using the contact process, i.e., the adhesives are applied to the substrate portion together with the carrier fluid in order to be able to develop good adhesion to the substrate portion. For this purpose, the film to be laminated is treated beforehand with a primer. During the lamination process itself, the film is subsequently heated and pressed by means of a vacuum onto a substrate portion. In this process, the adhesive located on the substrate joins with the primer of the film.
However, the disadvantage when using dissolved or dispersed adhesives is the necessity of having to apply the adhesive to the substrate portion. As a rule, the substrate portions are three-dimensionally molded. Therefore, it is necessary to spray the adhesive onto the molded substrate, which is relatively complicated in practice. In addition, the “overspray” arising can be critical, that is to say the fact that a portion of the respective adhesive compositions cannot be applied to the substrate portion.
To overcome this disadvantage, there has been an increase most recently in attempts to use polyurethane adhesives in the form of reactive hot melt adhesives for the film laminating process. When the latter are used, it is not necessary to spray the three-dimensionally molded substrate portion. Instead, it is sufficient to apply the adhesive to the film. This is considerably simpler, since the film is two dimensional. The adhesive can be applied easily by different processes as a film onto the film, and “overspray” can be avoided.
Reactive polyurethane hot melt adhesive compositions that are suitable for corresponding uses have already been described. They usually include isocyanate-terminated polyurethane prepolymers obtained by reacting suitable polyols with an excess of diisocyanates. Such adhesives develop a high initial strength immediately after their application due to cooling, and they achieve their final properties, for example, heat resistance and resistance to environmental influences, due to the gradually occurring “curing,” i.e., the chemical reaction of the isocyanate groups with atmospheric moisture. The reactive polyurethane adhesive has a distinctly lower molecular weight during processing than corresponding dissolved or dispersed polyurethane adhesives.
However, the polyurethane adhesives used today in laminating, regardless of whether they are applied dissolved, dispersed or as hot melt adhesive, have serious disadvantages in the case of use with olefinic substrates. Thus, as a rule, preliminary treatment of the corresponding substrates is necessary, since otherwise the polar polyurethane adhesive would not adhere to the nonpolar olefinic surface.
This disadvantage of highly different polar properties is considerably lower with adhesives based on amorphous poly-α-olefins in the case of use with olefinic substrates owing to the greater similarity of the properties of the adhesives to those of the films to be treated. In spite of this advantage, hardly any adhesive systems based on poly-α-olefins for laminating applications are encountered in practice. The reason for this is that olefinic adhesives marketed today are usually not suitable for vacuum deep-draw laminating or have other substantial disadvantages.
In addition to purely olefinic adhesives, ethylene/vinyl acetate copolymers modified with silane groups have also been described as adhesives. For example, GB 2197326 A discloses adhesive compositions made of silane-grafted ethylene/vinyl acetate copolymers with a vinyl acetate content in the range from 18 to 40% in combination with a catalyst and a plasticizer. It is exemplary for these adhesives to be characterized by a particularly advantageous resistance to organic solvents, a high adhesive strength, and a high resistance to creep failure of the adhesives. Some of the applications for such adhesives mentioned in GB 2197326 A are assembly, for example, in the furniture and car industry, packaging or labeling.
U.S. Pat. No. 5,731,384 also describes hot melt adhesives based on silane-modified ethylene-vinyl acetate which crosslink under the influence of moisture. It is exemplary for these adhesives to be characterized by good strength of adhesion to various substrates and advantageous adhesion properties at high temperatures, particularly on glass and aluminum.
Olefinic adhesives are available, for example, as dissolved systems; however, there is a demand for avoiding processing of solvent-containing adhesives. In addition, solvents that are suitable for dissolving nonpolar olefinic adhesives can be critical in terms of their use. Olefin-based hot melt adhesives, on the other hand, are commonly used, in particular in the car industry. Usually they are used as thermoplastic adhesives, but not for laminating three-dimensionally molded substrate portions. However, without chemical curing, the adhesive, when hot, does not have sufficient cohesion to the substrate portion in order to guarantee lasting stability.
In comparison to the adhesive technologies known to date, reactive polyolefin hot melt adhesives represent a relatively recent development, and they are not yet commonly used today for laminating in the car sector. Due to their chemical basis, the possibility of processing as a hot melt adhesive without carrier fluid, and the chemical secondary crosslinking by silane curing, this technology can be suitable for laminating olefinic substrates. Today, the first applications of olefinic hot melt adhesives are taking place in the vehicle sector, for example. However, in comparison to polar (for example, polyurethane) laminating adhesives, the available olefin adhesive solutions have the disadvantage that they develop very strong adhesion to uncoated and to Teflon-coated aluminum surfaces and are thus problematic in terms of their processing with corresponding tools.
Therefore, such adhesives are not suitable in the practical application of lamination. Laminated parts can be removed only with great difficulty from the tool after laminating. This makes utilization in practice difficult or impossible.
During lamination, particularly of films made of thermoplastic olefins such as polypropylene, the difficulty arises of the laminating hot melt adhesives developing adhesion to the laminating tool. In the case of vacuum deep-draw laminating processes, the adhesive, in this case the reactive hot melt adhesive, is applied to the back side of the decorative film and subsequently subjected to deep-draw lamination. For this purpose, the film together with the adhesive is heated and drawn onto a substrate portion, usually one based on polypropylene or resin-bound natural fibers. In the process, the molten adhesive comes in contact not only with the substrate portion to be laminated, but also with the laminating tool, in the area of the edges of the substrate portion, where the film becomes bonded with it.
An adhesive to be used in such a process therefore desirably does not adhere to the tool, since otherwise the component can subsequently no longer be removed directly from the tool. Below, this problem is referred to as a problem of adhesion. In order to avoid adhesion of the film, laminating tools are usually provided with repelling coatings. Teflon coatings are used commonly, since they have not only a good repelling action but also a long useful lifetime. Silicone coatings are also used in corresponding tools, but they are less common due to their lower useful lifetimes.
The disadvantage of such coatings is that the useful lifetimes can be of varying lengths depending on the coating type. The restoration of these coatings is associated with process downtimes and high costs.